1. Field of the Invention
The present invention relates to radio frequency (RF) receivers and more particularly to receivers capable of determining position information of satellites. It is particularly adaptable for use with global positioning satellite (GPS) systems. It utilizes common signal processing to minimize hardware circuitry.
2. Description of the Related Art
The Global Positioning System (GPS) consists of a constellation of satellites with synchronized atomic clocks that transmit radio signals. Time, as maintained by each satellite, is embedded in the transmitted radio signal of each satellite. The difference between the time embedded in a satellite's radio signal and a time measured at the point of reception of the radio signal by a clock synchronized to the satellite clocks is a measure of the range of the satellite from the point of reception.
Each satellite transmits, in addition to its clock time, its position in an earth-fixed coordinate system and its own clock error. A user, by measuring the pseudoranges to four satellites and correcting the pseudoranges for the satellite clock errors, can first of all determine his actual range to each satellite and his own clock error. The user can then determine his own position in the earth-fixed coordinate system, knowing his range to each of the four satellites and the position of each satellite in the earth-fixed coordinate system.
The current GPS system utilizes two signal frequencies for transmission. These frequencies are referred to as Link 1 (L1) and Link 2 (L2). The L1 signal frequency is 154fo (fo=10.23 MHz) or 1575.42 MHz and the L2 signal frequency is 120fo or 1227.6 MHz. These frequencies are modulated with pseudorandom sequences which are used to measure the time difference discussed above. Currently, there are two rates used for these sequences (10.23 MHz and 1.023 MHz). The higher rate is referred to as P/Y and the lower rate is referred to as C/A.
Several enhancements are planned to GPS. A third frequency (referred to as L5) is planned at 115fo or 1176.45 MHz with a PRN rate of 10.23 MHz. In addition, a new PRN sequence is also planned at 5.115 MHz for the existing L1 and L2 frequencies. The addition of these new signals will require additional processing in a GPS receiver. As will be disclosed below, the present invention proposes a method to process all the above GPS signals with common digital processing.
U.S. Pat. No. 6,369,753, issued to Schuker et. al., entitled “Host-Independent Monolithic Integrated Circuit for RF Downconversion”, discloses a global position system (GPS) receiver for a host product which is controlled by a microcontroller that also controls other functions in the host product. The GPS receiver includes an RF downconverter and a digital signal processor. The digital signal processor includes a correlator and an interface for asynchronously interfacing the correlator with the product's microcontroller. A monolithic integrated circuit includes RF downconverter circuitry, the correlator, and the interface for the GPS receiver
The Schuker et al system is primarily concerned with interfacing a GPS receiver to another system or subsystem and it is not concerned with military GPS signals or the new civil GPS signal (L5) or how these signals could be processed with common signal processing.
U.S. Pat. No. 5,663,734, issued to N. F. Krasner, entitled “GPS Receiver and Method for Processing GPS Signals”, discloses a GPS receiver in one embodiment that includes an antenna which receives GPS signals at an RF frequency from in view satellites; a downconverter coupled to the antenna for reducing the RF frequency of the received GPS signals to an intermediate frequency (IF); a digitizer coupled to the downconverter and sampling the IF GPS signals at a predetermined rate to produce sampled IF GPS signals; a memory coupled to the digitizer storing the sampled IF GPS signals (a snapshot of GPS signals); and, a digital signal processor (DSP) coupled to the memory and operating under stored instructions thereby performing Fast Fourier Transform (FFT) operations on the sampled IF GPS signals to provide pseudorange information. These operations typically also include preprocessing and post processing of the GPS signals. After a snapshot of data is taken, the receiver front end is powered down. The GPS receiver in one embodiment also includes other power management features and includes, in another embodiment the capability to correct for errors in its local oscillator which is used to sample the GPS signals. The calculation speed of pseudoranges, and sensitivity of operation, is enhanced by the transmission of the Doppler frequency shifts of in view satellites to the receiver from an external source, such as a base station in one embodiment of the invention.
The Krasner patent is primarily concerned with minimizing receiver power by powering down the RF section while digitally processing the signals and is not concerned with military GPS signals or the new civil GPS signal (L5) or how these signals could be processed with common signal processing.
Using a traditional approach, the GPS receiver would implement dedicated circuitry to tune the new signals. As will be disclosed below the present invention involves translating the existing and new GPS signal signals to a single new frequency using a combination of frequency mixing and critical sampling rates. This single intermediate frequency (IF) allows common digital circuitry to be used for all of the GPS signal processing.